The present disclosure relates to microfluidic test carriers for apportioning a quantity of fluid into sub-quantities.
Microfluidic elements for the analysis of a fluid sample are employed in diagnostic tests for in vitro diagnostics. In such tests, samples of body fluids are tested for one or more analytes contained therein, for medicinal purposes. An important component in the analysis is the test carrier, on which microfluidic channel structures are present for receiving and transporting a fluid sample in order to make it possible to carry out complex, multi-step tests (test protocols).
Test carriers, which are often referred to as a “Lab on a CD” or “Lab on a chip”, are made up of a carrier material, which is usually a substrate formed from a plastic. Examples of suitable materials are COC (cyclo-olefin-copolymer) or plastics such as PMMA (polymethyl methacrylate), polycarbonate or polystyrene. The test carriers have a channel structure that is formed in the substrate and is closed by a lid or a covering layer. The channel structure frequently consists of several channels and channel sections as well as interposed chambers that are broadened compared with the channels and channel sections. The structures and dimensions of the channel structures are defined by the structuring of the plastic parts of the substrates and may, for example, be produced by injection molding techniques or other appropriate methods. These also include methods that remove material, such as milling or the like.
Microfluidic test carriers are used, inter alia, for immunochemical analyses with a multi-step test procedure, for example enzyme-linked immunosorbent assay (ELISA) in which, for example, bound or free reaction components are separated. This requires controlled fluid transport. The procedure may be controlled by internal means (inside the fluid element) or external means (outside the fluid element). Control may be based on the use of pressure differences or changes in forces. Frequently, the test carrier is rotated in order to exploit centrifugal forces that are used to obtain control by changing the rotational speed, the direction of turning or the acceleration. Often, a combination of capillary and centrifugal forces are used to control the fluidics.
Analytical systems with rotary test carriers are known from the following publications, for example: European Pat. No. 0 626 071 B1; Int. Pat. Appln. Pub. No. WO 2007/042219 A1; Int. Pat. Appln. Pub. No. WO 01/46465 A2; Int. Pat. Appln. Pub. No. WO 95/33986; U.S. Pat. No. 5,160,702; and Int. Pat. Appln. Pub. No. WO 93/19827
An overview of microfluidic test elements and methods, control thereof, as well as microfluidic test elements as rotating disks, for example in the form of a compact disc (CD), is known from Marc Madou et al., Lab on a CD, 8 Annu. Rev. Biomed. Eng. 601-28 (2006) (online at bioeng.annualreviews.org).
Microfluidic test carriers often have a plurality of parallel sub-structures on one test carrier so that different analyses can be carried out in one process sequence. Distributing structures are provided in the test carriers to apportion the fluid into a plurality of identical or differently sized sub-volumes so that users are not obliged to apply small quantities of one sample fluid a number of times. These distributing structures also ensure that the significance of the results is not deleteriously affected by sample application or sampling effects. The same sample material is used for all analyses, which increases the significance, for example when carrying out multiple assays.
The prior art discloses, for example in U.S. Pat. No. 6,919,058 B2, distributing structures in which a fluid is accommodated in an elongated channel in the shape of a plurality of V-shaped structures arranged one behind the other in series. The distributing structure is positioned in a ring on a centrifuging platform. Venting capillaries are provided on the radially inward end of the leg of the V-shaped structure. Outlet capillaries are positioned at the radially outward part of the V-shaped structure, which are provided with a hydrophobic valve. Thus, the fluid is pre-distributed into the individual V-shaped structures on the principle of capillary forces. However, distribution of this type is very slow. After taking the fluid into the V-shaped distributing structure, the test carrier is rotated with acceleration so that the fluid breaks through the hydrophobic stop at a certain frequency and is drained off through the radially outwardly extending outlet capillaries at the base of the V-shaped structure. The fluid is partitioned at the moment of the valve breakthrough. Separation of the pre-distributed fluid portions occurs. Because of the architecture of the radially inwardly located part of the structure in particular, a predetermined volume is drained off. If the structures are the same size, then the volumes that are drained off are identical.
U.S. Pat. No. 4,154,793 discloses a rotary test carrier with a central receiving orifice in the lid. Below the orifice in the lid is a receiving chamber in which fluid is stored. Around the receiving zone are a plurality of peripheral sample chambers which are each connected to the receiving zone via a connecting channel. The inlet orifice for allowing fluid into the sample chamber is at a radially outwardly located position. In order to vent the sample chamber, an outlet orifice is provided that is positioned radially further inwardly than the inlet orifice and which connects the sample chamber to the receiving zone. Rotation of the test carrier empties the fluid contained in the receiving zone into the individual sample chambers, whereupon air flows out of the sample chambers into the receiving zone radially inwards and finally escapes through the central orifice in the lid.
U.S. Pat. No. 7,125,711 B2 discloses a test carrier with elongated distribution channel to which a plurality of metering chambers are connected, which latter are filled by capillary force. Each metering chamber comprises a sub-quantity and has an outlet with a geometric valve. Rotation of the test carrier causes the sub-quantities to empty out of the metering chambers into a sample analysis chamber.
The microfluidic distributing structures known from the prior art for apportioning a quantity of fluid into sub-volumes are particularly suitable for small volumes of up to approximately 10 μL, since the structures are filled entirely “passively” by capillary forces. This also makes the system highly dependent on the fluid properties of the sample, which compromises robustness. With larger volumes, distributing through extensive distributing structures results in significant time delays. This is because as the volumes become larger, the surface area-to-volume ratio of the capillaries becomes less and less favorable, which also results in a reduction in capillary force. In some cases, filling of the capillaries provided for the sub-quantities and the sub-quantity chambers may even come to a stop. Furthermore, with larger volumes, there is a risk that air will flow into the capillaries, which introduces errors into the apportioning and results in volume variations. In general, with distributing systems that function primarily by capillary action, trapped air and foam formation have a major influence on the precision of the sub-volumes that are drained off. This sensitivity of distributing systems that function by capillary action reduces the robustness of the processes (for example analytical processes). The lack of robustness has to be compensated for by external factors, for example by using automatic aliquoting robots. Thus, such known test carriers are only suitable for automatic pipetting of the fluids. Manual pipetting by different users, who are often pressed for time, has an increased tendency to result in (trapped) air bubbles and foam formation when pipetting. The known test carriers are not suitable for manual use. Since, in addition, with the capillary effects discussed here the surface properties of the process are dominant, manufacturing conditions and surface treatment conditions (for example activation, hydrophilization) play a major role. The tight tolerances increase the manufacturing and inspection costs for the test carrier during mass production and can also result in a high rejection rate.
Thus, there is still a great need in the art for the provision of a test carrier with which a quantity of fluid can be apportioned in a reliable manner into predetermined sub-quantities. Such a test carrier should not only be suitable for automatic pipetting and addition of the quantity of fluid, but should also be suitable for manual addition by different users and thus should exhibit enhanced robustness.